Four wheel drive system

ABSTRACT

A four wheel drive system for a vehicle includes comprising a power plant for producing a driving force, a front wheel driving mechanism for transmitting the driving force to drive right and left front wheels, a rear wheel driving mechanism for transmitting the driving force to drive right and left rear wheels, and right and left wheel clutches provided in one of the driving mechanisms for controlling the amount of the driving force transmitted to the wheels. A cut-off clutch is provided for controlling the driving force transmitted to the wheels of the one of the driving mechanisms, and a control device causes the cut-off clutch to engage before the wheel clutch is engaged. The driving force is transmitted sequentially rather than instantly from the cut-off clutch, the wheel clutches and to the wheels so as to suppress the torque shock creative in connection with a change in the drive condition.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention relates to a four wheel drive system for driving bothfront wheels and rear wheels of a vehicle by virtue of a power plant.

2. Description of Related Art

In four wheel drive vehicles, it has been known that a cut-off clutchmay be provided between the power plant and either a front wheel drivingsystem or a rear wheel driving system. The cut-off clutch is disengagedto establish a two wheel driving system when the four wheel drivecondition is not necessary to be maintained so that a power loss isreduced to improve fuel consumption efficiency.

Japanese Patent Public Disclosure No. 62-181916, laid open to the publicin 1987, proposes a four wheel drive system in which driving power istransmitted from the power plant to the rear wheels. The proposed fourwheel drive system is provided with hydraulic clutches (hereinafterreferred to as wheel clutches) on a driving mechanism for the right andleft rear wheels, respectively, to control driving torque for the rightand left rear wheels. As a result, torque distribution between the rightand left rear wheels is controlled to improve controllability of avehicle in a steering condition.

The wheel clutch can be provided in either the front or rear wheel drivemechanism.

Thus, appropriate torque distribution can be made between the front andrear drive system and/or between the right and left wheels to improvecontrollability.

It should be noted, however that a torque shock may occur when thecut-off clutch and the wheel clutches are engaged to transfer from thetwo wheel drive condition to the four wheel drive condition. This isbecause torque from the power plant is abruptly transmitted to thewheels.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to reduce the torque shockin the case where a switching operation is made between the two wheeldrive condition and the four wheel drive condition.

It is another object of the invention to reduce the torque shock whenthe wheel clutches are engaged.

It is still another object of the present invention to improvedurability of bearing devices and oil sealing devices and the like,which are employed in the four wheel drive system.

The above and other objects of the invention can be accomplished by afour wheel drive system of a vehicle comprising a power plant forproducing driving force, a front wheel driving mechanism fortransmitting the driving force to drive right and left front wheels, arear wheel driving mechanism for transmitting the driving force to drivethe right and left rear wheels, right and left wheel clutches providedin one of the driving mechanisms for controlling the amount of thedriving force transmitted to the wheels, a cut-off clutch forcontrolling the driving force transmitted to the wheels of the one ofthe driving mechanisms, and control means for engaging the cut-offclutch before the wheel clutch is engaged.

In another aspect of the invention, the cut-off clutch is temporarilyengaged at a predetermined time interval in the case where both thecut-off clutch and the wheel clutches are disengaged.

In preferred control, the control means disengages the cut-off clutchafter the wheel clutches are disengaged.

Further, the control means preferably disengages the wheel clutches whenan anti-lock braking system is in operation.

In a preferred embodiment, the control means increases a torquedistribution ratio for the rear wheels as speed difference between thefront and rear wheels is increased.

The control means may increase torque distribution ratio for the rearwheels as speed difference between the right and left wheels isincreased and may increase torque distribution ratio for the rear wheelsas vehicle speed is increased.

In a preferred embodiment, there is further provided a pair of controlvalves for driving the right and left wheel clutches. The control valvesare controlled by the control means to control hydraulic pressureintroduced into the wheel clutches so that engaging force of the wheelclutches are changed respectively.

In another preferred embodiment, a speed reduction mechanism isconnected with an input shaft of the one of the driving mechanisms forreducing a rotation speed of the input shaft transmitted to the wheelsdriven through the one of the driving mechanisms. The speed reductionmechanism includes a ring gear case in which the wheel clutches aredisposed.

There is provided an oil pump connected with and driven by the inputshaft of the one of the driving mechanisms for producing the hydraulicpressure for the wheel clutches. A casing is provided for receiving thering gear case of the speed reduction mechanism and the oil pump.

According to the present invention, in the case where the drivecondition is changed from the two wheel drive condition to the fourwheel drive condition, the cut-off clutch is engaged before the wheelclutch is engaged. On the contrary, when the cut-off clutch isdisengaged after the wheel clutches are disengaged. Thus, the drivingforce is transmitted not instantly but sequentially from the cut-offclutch, the wheel clutches and thereafter to the wheels so as tosuppress the torque shock of the vehicle in connection with a change inthe drive condition.

Alternatively, the cut-off clutch is temporarily engaged atpredetermined intervals so that the driving mechanism having the wheelclutches can get enough lubrication for bearing and oil sealing and thelike provided therein. Further, a compact hydraulic control system isprovided for controlling the wheel clutches.

The above and other objects and features of the present invention willbe apparent from the following description making reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a four wheel drive system in accordancewith a preferred embodiment of the present invention;

FIG. 2 is a flow chart of a main control program for a driving forcedistribution;

FIG. 3 is a flow chart of a clutch control in accordance with thepreferred embodiment;

FIG. 4 and FIG. 5 are graphical representations showing a relationshipbetween a speed difference and torque distribution ratio;

FIG. 6 is a graphical representation showing a relationship betweentorque distribution and vehicle speed;

FIG. 7 is a graphical representation showing a relationship between basetorque distribution ratio and steering angle change rate;

FIG. 8 is a graphical representation showing a relationship between aconstant for compensating the distribution ratio and the steering angle;

FIG. 9 is a time chart of steering angle and other variables;

FIG. 10 is a flow chart of clutch engagement control;

FIG. 11 is a sectional view of a rear wheel drive mechanism inaccordance with another embodiment of the present invention;

FIG. 12 is a sectional view of a right rear axle in accordance with theembodiment of FIG. 11;

FIG. 13 is a sectional view of an oil pump in accordance with theembodiment of FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a schematic view of a four wheel drive system provided witha control system therefor according to a preferred embodiment of thepresent invention.

Referring to FIG. 1, an illustrated four wheel drive vehicle is providedwith a power plant 3 constituted by an engine 1 and a transmission 2, afront wheel drive mechanism 10 for driving right and left front wheels6, 7 and a rear wheel drive mechanism 20 for driving a right and leftrear wheels 8, 9 to which a driving force produced by the power plant 3is transmitted through a pair of power transmitting gears 4, 5.

The front wheel drive mechanism 10 extends in a longitudinal directionof the vehicle and is provided with a front drive shaft 11 driven by thedriving force of the power plant 3 transmitted through the gears 4 and5, a differential mechanism 12 for sharing the driving force through thefront axle 11, a right axle 14 and left axle 13 to which the drivingforce is allotted through the differential mechanism 12 and whichtransmit the allotted driving force to the right and left wheels 6, 7 sothat the front wheels 6 and 7 are always driven by the driving force ofthe power plant.

The rear wheel drive mechanism 20 extends in a longitudinal direction ofthe vehicle and is provided with a rear drive shaft 21 driven by thedriving force of the power plant 3 transmitted through the gears 4 and5, a right axle 25 and left axle 24 which are driven by the drive shaft21 through bevel gears 22, 23 and transmit the driving force to theright and left wheels 9, 8 so that the front wheels 6 and 7 are alwaysdriven by the driving force of the power plant.

On the rear drive shaft 21 is disposed a cut-off clutch 26 forcontrolling the driving force transmitted to the rear axles 24, 25 orthe rear wheels 8 and 9. On the rear axles 24 and 25 are disposed wheelclutches 27 and 28 in the form of multi-plate hydraulic clutches forcontrolling the driving force transmitted to the rear wheels 8 and 9.The wheel clutches 27 and 28 are provided with a plurality of clutchplates driven by a hydraulic pressure which is introduced from ahydraulic pressure source such as an oil pump through oil passages 31,32 and controlled by pressure control valves 33, 34.

A controller 35 is provided for controlling the cut-off clutch 26 andthe control valves 33 and 34. The cut-off clutch 26 is controlled, by acontrol signal a produced by the controller 35, to be disengaged andengaged.

The control valves 33 and 34 with solenoids are controlled by signals b,c from the controller 35 to control the hydraulic pressure for the wheelclutches 27 and 28 so that torques transmitted to the wheel clutches 27and 28 are changed continuously.

The controller 35 receives signals d, e, f and g from wheel speedsensors 36-39 for detecting wheel rotation speed of the wheels 6-9, asignal h from a steering sensor 41 for detecting a steering amount ofsteering wheel 40, a signal i from a lateral acceleration sensor 42 fordetecting lateral acceleration acting on the vehicle body and a signal jfrom an ABS controller 43 for controlling an anti-lock braking system.The controller 35 controls the cut-off clutch 26 and the right and leftclutches 28, 27 based on the signals d-j.

Referring to FIG. 2, there is shown a flow chart of a main routine bythe controller 35. In step S1 the controller 35 initializes the systemincluding variables and flags employed in the control. In step S2, thecontroller 35 determines a torque distribution ratio between the rightand left rear wheels 9, 8. In step S3, the controller 35 produces thesignals b and c to the hydraulic pressure control valves 33 and 34 foraccomplishing the torque distribution determined by the step S2 andproduces the signal a for disengaging and engaging the cut-off clutch26.

The hydraulic pressure for the clutches 27 and 28 are controlled inaccordance with a subroutine shown in FIG. 3 in the form of the flowchart.

According to the hydraulic pressure control for the clutches 27 and 28,the controller 35 obtains the wheel rotation speeds ω_(FL), ω_(RF),ω_(RL), ω_(RR), the steering angle Θ, the steering angle change rate dΘ,the vehicle speed V and the lateral acceleration G, through the signalsfrom the sensors 36-39, and ABS signal F_(ABS) through the signal j fromthe ABS controller 43 which indicates if the ABS controller 43 is in theABS control. In this case, the steering angle change rate d Θ can becalculated based on the steering angle Θ. The vehicle speed is providedas a slowest value among the wheel speeds ω_(FL), ω_(FR), ω_(RL),ω_(RR).

Next, the controller 35 judges the value of the ABS signal F_(ABS). Whenthe ABS signal F_(ABS) is 1, in other words, when the ABS control isbeing carried out, both the torque distribution T_(RL) for the left rearwheel 8 and the torque distribution T_(RR) for the right rear wheel 9are set at 0. In this case, the hydraulic pressure is not introducedinto the wheel clutches 27 and 28 and they are disengaged. Thus, thedriving force from the power plant 3 is not transmitted to the rearwheels 8 and 9. As a result, the rear wheels 8 and 9 can be rotatedfreely to accomplish a desirable ABS control.

On the other hand, when the ABS control is not being carried out, thevalue of the ABS signal F_(ABS) =0. In this case, the controller 35executes steps S12 to S14 and judges whether or not the absolute valueof the steering angle Θ is smaller than a predetermined value Θ₀. Thepredetermined value Θ₀ denotes the size of dead zone for the steeringangle. Thus, when the absolute value of the steering angle Θ is withinthe predetermined valueΘ₀, it is judged that there occurs no steeringoperation, that is, the vehicle runs on a straight path.

When the vehicle is in the straight path running condition, thecontroller 35 resets values of cornering and timer flags F_(C). F_(TM)in step S15. In step S16, a speed difference Δω₁ between the front andrear wheels is calculated based on the following equation;

    Δω.sub.1 =(ω.sub.FL +ω.sub.FR)-(ω.sub.RL +ω.sub.RR)

In step S17, the controller 35 obtains a first rear torque distributionratio T₁ based on the speed difference Δω₁ in light of a map shown FIG.4. The first rear torque distribution ratio T₁ is used for obtaining thefinal torque distribution ratios T_(RL),T_(RR). In this case, as thespeed difference Δω₁ takes a positive value and is increased. In otherwords, as the rotation speed of the front wheels is greater than that ofthe rear wheels, the value of the first rear torque distribution ratioT₁ is increased as shown in FIG. 4. Thus, if slippage of the frontwheels 6, 7 is increased because of a greater torque distributionthereto as compared with the rear wheels 8, 9, the torque distributionfor the rear wheels 8, 9 is increased to reduce the slippage of thefront wheels 6, 7.

In step S18, the controller 35 obtains a speed difference Δω₂ betweenthe right wheels 7, 9, and left wheels 6, 8.

    Δω.sub.2 =(ω.sub.FL +ω.sub.RL)-(ω.sub.FR +ω.sub.RR)

In step S19, the controller 35 obtains a second rear torque distributionratio T₂ based on the absolute value of the speed difference Δω₂ inlight of a map shown in FIG. 5. The second rear torque distributionratio T₂ is used for obtaining the final torque distribution ratiosT_(RL),T_(RR). In this case, as the absolute value of the speeddifference Δω₂ is increased, the second rear torque distribution ratioT₂ is increased. Thus, when the speed differenceΔ ω₂ between the rightwheels 7, 9, and left wheels 6, 8 is increased, the torque distributionfor the rear wheels 8, 9 is increased to improve running stability ofthe vehicle.

In step S20, the controller 35 obtains a third rear torque distributionratio T₃ in accordance with the vehicle speed V in light of a map shownin FIG. 6. The third rear torque distribution ratio T₃ is also used forobtaining the final torque distribution ratios T_(RL) , T_(RR). In thiscase, as the vehicle speed V is increased, the third rear torquedistribution ratio T₃ is increased as shown in FIG. 6. Thus, when thevehicle speed V is increased, the torque distribution for the rearwheels 8, 9 is increased to improve running stability of the vehicle ona straight path.

In step S21, the controller 35 selects the maximum value of the first,second and third torque distribution ratios T₁, T₂ and T₃ as a totaltorque distribution ratio T_(R) for the rear wheels 8, 9. In step S22,the controller 35 allots the torque distribution ratio T_(R) to theright and left rear wheels 8, 9 as the torque distribution ratiosT_(RL), T_(RR).

Thus, when the speed differences Δω₁, Δω₂ are zero and the vehicle speedis not high, all of the first, second and third torque distributionratios T₁, T₂ and T₃ become zero so that there is no torque distributionfor the rear wheels 8, 9 to establish the two wheel drive condition.When one or more values of the first, second and third torquedistribution ratios T₁, T₂ and T₃ is not zero, the driving force of thepower plant 3 is transmitted to the rear wheels 8, 9 to establish thefour wheel drive condition. When the four wheel drive condition isestablished in the straight path running condition, the torque isequally distributed to the right and left rear wheels 9, 8.

According to the illustrated embodiment, when one of the torquedistribution ratios T₁, T₂ and T₃ takes a value greater than the maximumvalue 0.5, the torque is equally distributed to the respective front andrear wheels. Thus, each wheel of the vehicle gets a quarter of thedriving force produced in the power plant 3.

On the other hand, when the steering angle Θ is greater than thepredetermined valueΘ₀, that is, when the vehicle is in a corneringcondition, the controller 35 carried out steps S14-S23 and judges thevalue of the flag F_(c). The flag F_(c) takes a value of zero when thevehicle runs on a straight path or when the steering angle Θ is beingincreased at an initial stage of the cornering condition. The flag F_(c)takes a value of 1 when the steering angle Θ is substantially constant,that is, when the vehicle is in a stable cornering condition. Thus, thecontroller 35 carries out steps S23 and S24 since the flag F_(c) takes avalue zero when the vehicle is in a transitional condition from thestraight path running condition to the cornering condition. Thecontroller 35 judges whether or not the absolute value of the steeringangle change rate dΘ is greater than zero in step S24. When the vehicleis in an initial stage of the cornering condition, the steering anglechange rate d Θ is greater than zero. In this case, the controller 35carries out step S25. The controller 35 sets the maximum value max(dΘ)at the current steering angle change rate d Θ.

In step S26, the controller 35 sets base distribution ratios T_(RLB),T_(RRB) for the left and right rear wheels 8, 9 based on a map shown inFIG. 7. When the steering operation is applied on the steering wheel 21in a counterclockwise direction, the steering angleΘ is defined aspositive. Thus, if the steering operation is made in thecounterclockwise direction, the base distribution ratio T_(RRB) for theright rear wheel 9 is greater than the ratio T_(RLB) for the left rearwheel 8. On the contrary, when the steering wheel 21 is steered in theclockwise direction, the ration T_(RLB) for the left rear wheel 8 isgreater than the ratio T_(RRB) for the right rear wheel 9. As thesteering angle change rate dΘ is increased, the base distribution ratiosT_(RLB), T_(RRB) for the left and right rear wheels 8, 9 are increased.In step S27, the final torque distribution ratios T_(RL), T_(RR) arereplaced by the base distribution ratios T_(RLB) for the left and rightrear wheels 8, 9.

As a result, as the steering angle change rate dΘ is increased (stageA), the final torque distribution ratios T_(RL), T_(RR) are increased aswell as shown in FIG. 9. The final torque distribution ratios T_(RL),T_(RR) are maintained at the maximum value (T_(RL))_(MAX),(T_(RR))_(MAX)of the final torque distribution ratios which correspond to the maximumvalue of max(dΘ) during the period while the steering angle change ratedΘ is reduced to zero after the value dΘ takes the maximum vale max(dΘ)(stage B), that is, while the steering angle Θ reaches a peak value andtakes a substantially constant value Θ₁. As aforementioned, the greatertorque distribution is made for the rear wheels 8, 9 until the vehiclereaches the stable cornering condition. In this case, the outer rearwheel of the cornering action gets more torque distribution than theinner rear wheel. As a result, the vehicle can get an improvedcontrollability during the initial cornering action.

Thereafter, when the vehicle reaches a constant or stable corneringcondition wherein the steering angle Θ is substantially constant, theflag F_(c) is set at 1 is step S28. The controller 35 carries out stepsS23-S29 and judges the value of the timer flag F_(TM). The flag F_(TM)takes a value of 0 initially. Thus, the controller 35 sets a constant C₀for reducing the torque distribution ratio for the rear wheels 8, 9 instep S30. The constant C₀ is determined based on a map shown in FIG. 8.As the absolute value of the constant steering angleΘ₁ is increased, theconstant C₀ is decreased. In step S31, the timer flag F_(TM) is setat 1. A decrement C of the torque distribution ratio is rest at 0. Instep S32, a decrement coefficient K is calculated by using the decrementC based on the following equation;

    K=(1000-C)/1000

The number 1000 is illustrative. The constant C₀ can take any value lessthan 1000 in this embodiment.

The decrement coefficient K is provided as a value of 1 initially.Thereafter, the value K is reduced, since the decrement C is increasedby the constant C₀ in each cycle in steps S29-S33. When it is found thatthe value K is reduced below zero, the value K is fixed at zero.

The controller 35 calculates the final torque distribution ratiosT_(RL), T_(RR) by multiplying the value K which changes between 1 and 0into the base distribution ratios T_(RLB) (=(T_(RL))_(MAX)), T_(RRB)(=(T_(RR))_(MAX)) in step S36.

As aforementioned, the final torque distribution ratios T_(RL), T_(RR)is decreased from the maximum values (T_(RL))_(MAX), (T_(RR))_(MAX) inthe stage B to zero during the constant cornering condition (stage C) asshown in FIG. 9. This means that the vehicle is gradually changed fromthe four wheel drive condition, to the two wheel drive condition inwhich only the front wheels are driven, so that a stable corneringproperty can be obtained in the constant cornering condition.Specifically, the constant C₀ is decreased as the constant steeringangle Θ₁ is increased. As a result, the torque distribution ratiosT_(RL), T_(RR) are gradually reduced to zero. As seen by a phantom linein FIG. 9, it takes longer as the steering angle Θ₁ is decreased toaccomplish a smooth transition from the four wheel drive condition tothe two wheel drive condition.

After determining the torque distribution ratios T_(RL), T_(RR) for therear wheels 8, 9, the controller 35 produces the control signals b, cfor the pressure control valves 33, 34 for controlling on engaging levelor engaging force of the wheel clutches 27, 28 to accomplish the torquedistribution ratios T_(RL), T_(RR) and controls the cut-off clutch 26 bythe signal a in accordance with a routine as shown in FIG. 10 in theform of a flow chart. The engaging force of the wheel clutches 27, 28are changed continuously with the hydraulic pressure introducedthereinto. The hydraulic pressure is continuously controlled by thecontrol valves 33, 34 having solenoids controlled by the signals b, c ofthe controller 35.

In FIG. 10, in step S41, the controller 35 judges a value of anabnormality flag F_(TRB) which takes a value of 1 when both of the wheelclutches 27, 28 are out of order. In this case, the controller 35disengages the cut-off clutch in step S42. Thus, when both of the wheelclutches 27, 28 are out of order, the driving force is not transmittedto the rear wheels 8, 9 so that the two wheel drive condition by thefront wheels is established.

If at least one of the wheel clutches 27, 28 are normal and thus theflag F_(TRB) =0, the controller 35, in step S43, judges the value of theinitial running flag F_(INI) which takes a value of 0 until the runningdistance of the vehicle reaches a predetermined value such as 300 mafter starting and takes a value of 1 after the running distance reachesthe predetermined value. Therefore, the flag F_(INI) is zero at thebeginning so that the controller 35 goes to steps S43 and S44. When theengine is started, the controller 35 carries out step S45 and engagesthe cut-off clutch 26. When the running distance exceeds thepredetermined value (300 m in this embodiment), the controller 35carries out steps S46, S47 and sets the flag F_(INI) at 1.

Therefore, before the running distance reaches the predetermined value,the cut-off clutch 26 is kept on the engaged condition regardless of thecondition of the wheel clutches 27, 28.

When the running distance exceeds the predetermined value after startingand thus the flag F_(INI) is changed from 0 to 1, the controller 35carries out step S48 and judges the value of the torque distributionratios T_(RL), T_(RR) determined by the control shown in FIG. 3. Whenboth of the torque distribution ratios T_(RL), T_(RR) are zero, that is,the vehicle is in the two wheel drive condition, the cut-off clutch 26is disengaged in step S49. Thus, the driving force is no longertransmitted to a driven side of the clutch 26 including the rear wheeldrive mechanism 20.

When at least one of the torque distribution ratios T_(RL), T_(RR) arenot zero, the controller 35 carries out step S50 to judge whether or notthe cut-off clutch 26 is engaged. If the cut-off clutch 26 isdisengaged, the controller 35 is caused to engage the cut-off clutch 26by the control signal in step S51. If the cut-off clutch 26 is engaged,the controller 35 holds as it is. In step S52, the controller 35produces the signals b, c to engage the wheel clutches 27, 28. Thus,when the four wheel drive condition is established, the cut-off clutch26 is engaged and thereafter, the wheel clutches 27, 28 are engaged toaccomplish the torque distribution ratios T_(RL), T_(RR) for the rearwheels 8, 9.

In step S53, the controller 35 judges whether or not both of the wheelclutches 27, 28 are out of order. In the case where both of the wheelclutches 27, 28 are out of order, the controller 35 disengages thecut-off clutch 26 and sets the flag F_(TRB) at the value 1 in steps S54,S55.

As aforementioned, in the case where the wheel clutches 27, 28 aredisengaged, that is, where the torque distribution ratios T_(RL), T_(RR)are zero, the cut-off clutch 26 is disengaged (step S49) so that thedriven side of the drive shaft 21, the pair of bevel gears 22, 23, drivesides of the axles 24, 25 and drive sides of the wheel clutches 27, 28will not be driven unnecessarily. Thus, the loss of the driving forcecan be saved.

Moreover, in the case where the wheel clutches 27, 28 are caused to beengaged, the cut-off clutch 26 is engaged prior to the actual engagementof the wheel clutches 27, 28. Therefore, the driving force from thepower plant 3 is sequentially transmitted to the rear wheels 8, 9 sothat the torque shock caused by the engagement of the wheel clutches 27,28 can be obviated.

According to the above control, the cut-off clutch 26 is engaged in theinitial running until the running distance reaches the predetermineddistance (300 m) after starting so that movable parts, such as the driveshaft 21, the pair of bevel gears 22, 23, drive sides of the axles 24,25 and drive sides of the wheel clutches 27, 28 and the like are drivenin appropriate intervals. As a result, bearing devices, oil sealings andthe like associated with the movable parts can be lubricated. Thecut-off clutch 26 can be temporarily engaged during any runningcondition other than the initial running condition so as to provide themovable parts driven by the cut-off clutch 26 with enough lubrication.

Referring to FIGS. 11-13, there is shown another embodiment of thepresent invention, specifically, another structure of the powertransmitting mechanism. According to the second illustrated embodiment,the power transmitting mechanism is employed as a rear wheel drivemechanism 20. This mechanism is used to replace the bevel gears 22, 23and the wheel clutches 27, 28 in the former embodiment.

The rear wheel drive mechanism 20 is provided with a casing 45 in whichthe left rear axle 24 and right rear axle 25 are rotatably carried. Oilsealings 47, 49 are disposed between the casing 45 and the axles 24, 25for sealing.

Within the casing 45, a ring gear case 44 is rotatably mounted on thecasing 45. The ring gear case 44 includes a cup like member 46 facingthe right axle 24 and lid like member 50 facing the left axle 25 andjoined with the cup like member 46 through a bolt 48. The lid likemember 50 is rotatably carried by the casing 45 through a roller bearingmechanism 52. The member 50 rotatably carries the left rear axle 24. Thecup like member 46 is rotatably carried by the casing 45 through aroller bearing mechanism 54. The member 46 rotatably carries the rightrear axle 25.

An input shaft 56 of the rear wheel drive mechanism 20 joined with thedrive shaft 21 at a front end is rotatably carried by the casing 45. Theinput shaft 56 extends in a direction perpendicular to the axles 24, 25.The input shaft 56 is formed at a rear end with a drive pinion gear 58which is meshed with a ring gear 62 fixed to the ring gear case 44 bymeans of a bolt 60. The input shaft 56 and the ring gear case 44constitute a speed reduction mechanism.

There is provided a hydraulic clutch mechanism for transmitting therotation of the ring gear case 44 to the left and right rear shafts 24,25 independently.

With the cup like member 46 of the ring gear case 44 is engaged an outerside of a peripheral portion 66 of a support assembly 64 through aspline mechanism 68 formed thereon. The support assembly 64 rotatablycarries the axle 25. An inner side of the peripheral portion 66 isengaged with plurality of left clutch plates 72 and right clutch plates74 through the spline mechanisms 76 and 78. The axles 24, 25 are engagedwith ring members 80, 82 through the spline mechanisms 83, 83' at outersurfaces. Outer sides of the ring members 80, 82 are engaged with aplurality of left clutch plates 84 and right clutch plates 86 throughspline mechanisms 88, 90. The clutch plates 84, 86, 72 and 74 constitutea left hydraulic clutch 92 and a right hydraulic clutch 94. In order tooperate the hydraulic clutches 92, 94, the support assembly 64 isprovided with a left hydraulic pressure unit 96 and a right hydraulicpressure unit 98. The hydraulic pressure units 96, 98 are provided withpistons 100, 102, hydraulic chambers 104, 106 formed between the pistons100, 102 and the support assembly 64. Numerals 108, 110, 112 and 114designate oil sealings. There are disposed retainers 116, 118 at anouter side of the inner portion 70 and springs 120, 122 between thepistons 100, 102 and the retainers 116, 118. The pistons 100, 102 areurged toward an initial position. The spring 120, 122 is disposed aroundshafts 124, 126 on which the retainers 116, 118 are mounted. Steel balls132, 134 are disposed in oil passages 128, 130, communicated withhydraulic chambers 104 and 106, to form check valves which prevent oilflow out of the chambers 104, 106 and allow oil flow to the chambers104, 106. When hydraulic pressure is not introduced into the chambers104, 106, the pistons 100, 102 are positioned at the initial positionsby virtue of the springs 120, 122 so that the hydraulic clutches 92, 94are disengaged. Thus, the rotation of the ring gear case 44 transmittedfrom the input shaft 56 is not transmitted to the left and right axles24, 25.

When the hydraulic pressure is introduced into the chambers 104, 106,the pistons 100, 102 are moved against the springs 120, 122 to engagethe hydraulic clutches 92, 94. As a result, the rotation of the ringgear case 44 transmitted from the input shaft 56 is transmitted to theleft and right axles 24, 25.

When the hydraulic pressure is introduced into one of the chambers 104,106, the driving force is transmitted to the one of the clutches 92, 94to drive one of the axles 24, 25 or the wheels 8, 9.

Hereinafter, there is described a hydraulic pressure introducingmechanism for the oil chambers 104, 106.

There is provided an oil pump 136 which is driven by the rotation of theinput shaft 56. The hydraulic pressure generated in the oil pump 136 isintroduced into the right and left control valves 34 and 33 which areduty solenoid valves through an oil passage 138.

Thereafter, the hydraulic pressure is introduced into oil passage 148for the left hydraulic clutch 92 and oil passage 150 for the righthydraulic clutch 94. Then the hydraulic pressure of the oil passage 148is introduced into the left oil chamber 104 through annular recess 152,radial oil passage 154, axial oil passage 156, radial oil passage 158,annular recess 160 formed in the right axle 25, and an oil passage 162formed in the support assembly 64. The annular recesses 152 and 160 areformed in the right axle 25 so that the oil passage 148 can be alwayscommunicated with the radial oil passage 154 through the annular recess152 and the radial oil passage 158 is always communicated with the oilpassage 162 as the axle 25 is rotated relative to the casing 45 and thesupport assembly 64.

Likewise, the hydraulic pressure of the oil passage 150 is introducedinto the oil chamber 106 through annular recess 164, radial oil passage166, axial oil pressure 168, annular oil passage 172 formed in the rightaxle 25, and oil passage 174 of the support assembly 64.

When the control valves 33, 34 are controlled independently, thehydraulic pressure of the chambers 104 and 106 can be controlledindependently.

The oil pump 136 is mounted on an end portion 176 of the casing 45. Anoil sealing 178 is disposed between the end portion 176 of the casing 45and the oil pump 136 for sealing. Numeral 180 designates an oil returnpassage for returning the oil in the casing 45 to the oil pump 136.

Referring to FIG. 4, FIGS. 11 and 13 the pump 136 is of a trochoidalconfiguration and is provided with a pump housing 182, an outer ringassembly 186 rotatably disposed in the pump housing 182 and formed withinner teeth 184 and an inner ring assembly 190 rotatably disposed in theouter ring assembly 186 and formed with outer teeth 188. The outer ringassembly 186 has a different rotation axis from the inner ring assembly190 so that the inner teeth 184 of the outer ring assembly 186 arepartly meshed with the outer teeth 188 of the inner ring assembly 190. Aflange member 192 is engaged with the input shaft 56 through a sprinemechanism 194. The flange 192 is engaged with the inner ring assembly190 through a steel ball 196.

Since the flange 192 is rotated integrally with the input shaft 56, theinner ring assembly 190 is also rotated to rotate the outer ringassembly 186. This rotation of the inner and outer ring assemblies 186and 190 produces the hydraulic pressure which is introduced into the oilpassage 138.

Roller bearing mechanisms 198, 199 are provided within the casing 45 forrotatably carrying the input shaft 56. The roller bearing mechanism 198is provided with a support member 200 mounted on the casing 45, asupport member 202 mounted on the input shaft 56 and roller bearings 204between the members 200 and 202. Likewise, the roller bearing mechanism199 is provided with a support member 208 mounted on the casing 45, asupport member 210 mounted on the input shaft 56 and roller bearings 212between the members 208 and 210. The support members 202 and 210 aremovable for adjustment in an axial direction of the input shaft 56. Aspring 214 is disposed between the support members 202 and 210 in theaxial direction for urging them. The support member 210 abuts against astopper 218 at a right end portion 216 so that further rightwardmovement is restricted.

When the input shaft 56 is positioned relative to the casing 45, a bolt222 at a front end portion 220 thereof is released so that the flange192 can be moved relative to the input shaft since they are engaged witheach other through the spline mechanism 194. This axial movement of theflange 192 causes the support member 202 to be moved in the axialdirection of the input shaft 56 against and with the resilient force ofthe spring 214.

As aforementioned, the axial position of the support member 202 isadjusted so that the input shaft 56 is axially positioned relative tothe casing 45. As a result, the drive pinion gear 58 of input shaft 56can be positioned to be engaged with the ring gear 62 of the ring gearcase 44 appropriately. The flange 192 is engaged with the inner ringassembly 190 of the oil pump 190 through the steel ball 196. A recess224 of the inner ring assembly 190 is extended in the axial direction ofthe input shaft 56. Thus, as the flange 192 is moved axially, the steelball can be moved axially within the recess 224 of the inner ringassembly 190 to allow relative movement of the pump 136 and the flange192. In other words, the axial movement of the flange 192 does notaffect the pump badly.

In the above embodiment, although the drive mechanism is applied to therear wheel drive mechanism, the above mechanism can be similarly appliedto the front wheel drive mechanism 10. The driving force of the powerplant 3 can be controlled for the respective wheels 6, 7, 8 and 9through the control of the control valves 33, 34 in combination with thecontrol of the clutch 26 as well as the former embodiment so that thesame effect can be obtained as the embodiment.

It should be noted that although the present invention is described inconnection with a specific embodiment making reference to theaccompanying drawings, many modifications can be made by the thoseskilled in the art based on the foregoing, and all of modifications areintended to be included within the scope of the present invention asdefined by the attached claims.

What is claimed is:
 1. A four wheel drive system of a vehiclecomprising:a power plant for producing a driving force, a front wheeldriving mechanism for transmitting the driving force to drive right andleft front wheels, a rear wheel driving mechanism for transmitting thedriving force to drive right and left rear wheels, right and left wheelclutches provided in one of the driving mechanisms for controlling theamount of the driving force transmitted to wheels driven through the oneof the driving mechanisms, said right and left wheel clutches beingcontrolled to disengage the wheels driven through the one of the drivingmechanisms in a predetermined disengaging condition, a cut-off clutchfor controlling the driving force transmitted to the wheels driventhrough the one of the driving mechanisms, and control means forengaging the cut-off clutch before the right and left wheel clutches arerestored to an engaged condition from said predetermined disengagingcondition.
 2. A four wheel drive system as recited in claim 1 whereinthe control means disengages the cut-off clutch after the right and leftwheel clutches are disengaged.
 3. A four wheel drive system as recitedin claim 1 wherein the control means disengages the right and left wheelclutches when an anti-lock braking system is in operation.
 4. A fourwheel drive system as recited in claim 1 wherein the control meansincreases a torque distribution ratio for the rear wheels as a speeddifference between the front wheels and the rear wheels is increased. 5.A four wheel drive system as recited claim 1 wherein the control meansincreases a torque distribution ratio for the rear wheels as a speeddifference between the right wheels and the left wheels is increased. 6.A four wheel drive system as recited in claim 1 wherein the controlmeans increases a torque distribution ratio for the rear wheels asvehicle speed is increased.
 7. A four wheel drive system as recited inclaim 1 wherein the cut-off clutch is temporarily engaged at apredetermined time interval when both the cut-off clutch and the rightand left wheel clutches are disengaged.
 8. A four wheel drive system asrecited in claim 1 wherein each of the right and left wheel clutches isa multi-late hydraulic clutch operated by a hydraulic pressure whichchanges engaging forces of the right and left wheel clutchescontinuously.
 9. A four wheel drive system as recited in claim 1, andfurther comprising a pair of control valves for driving the right andleft wheel clutches, the control valves being controlled by the controlmeans to control hydraulic pressure introduced into the right and leftwheel clutches so that respective engaging forces of the right and leftwheel clutches are changed.
 10. A four wheel drive system as recited inclaim 9 wherein each of the control valves is a solenoid valvecontrolled by a control signal from the control means.
 11. A four wheeldrive system as recited in claim 1 and further comprising a speedreduction mechanism connected with an input shaft of the one of thedriving mechanisms for reducing a rotation speed of an input shafttransmitted to the wheels driven through the one of the drivingmechanisms.
 12. A four wheel drive system as recited in claim 11 whereinthe speed reduction mechanism comprises a ring gear case in which thewheel right and left clutches are disposed.
 13. A four wheel drivesystem as recited in claim 12, and further comprising a pair of controlvalves controlled by the control means for controlling a hydraulicpressure introduced into the right and left wheel clutches so thatengaging forces of the right and left wheel clutches are changed,respectively, and an oil pump connected with and driven by an inputshaft of the one of the driving mechanisms for producing the hydraulicpressure for the right and left wheel clutches.
 14. A four wheel drivesystem of a vehicle comprising:a power plant for producing a drivingforce, a front wheel driving mechanism for transmitting the drivingforce to drive right and left front wheels, a rear wheel drivingmechanism for transmitting the driving force to drive the right and leftrear wheels, right and left wheel clutches provided in one of thedriving mechanisms for controlling the amount of the driving forcetransmitted to wheels driven through the one of the driving mechanisms,a cut-off clutch for controlling the driving force transmitted to thewheels driven through the one of the driving mechanisms, control meansfor engaging the cut-off clutch before the right and left wheel clutchesare engaged, a speed reduction mechanism connected with an input shaftof the one of the driving mechanisms for reducing a rotation speed of aninput shaft transmitted to the wheels driven through the one of thedriving mechanisms, the speed reduction mechanism comprising a ring gearcase in which the right and left wheel clutches are disposed, a pair ofcontrol valves controlled by the control means for controlling ahydraulic pressure introduced into the right and left wheel clutches sothat engaging forces of the right and left wheel clutches are changed,respectively, and an oil pump connected with and driven by an inputshaft of the one of the driving mechanisms for producing the hydraulicpressure for the right and left wheel clutches, and a casing in whichthe ring gear case and the oil pump are disposed.
 15. A four wheel drivesystem for a vehicle comprising:a power plant for producing a drivingforce, a front wheel driving mechanism for transmitting the drivingforce to drive right and left front wheels, a rear wheel drivingmechanism for transmitting the driving force to drive right and leftrear wheels, right and left wheel clutches provided in one of the frontand rear wheel driving mechanisms for controlling an amount of thedriving force transmitted to wheels driven through the one of the frontand rear wheel driving mechanisms, a cut-off clutch interposed betweenthe power plant and the right and left wheel clutches for controllingthe driving force transmitted to the wheels driven through the one ofthe front and rear wheel driving mechanisms, and control means forengaging the cut-off clutch before the right and left wheel clutches areengaged.